Wiktionary
n. A very thin strap on a dress, etc.
Wikipedia
A spaghetti strap (also called noodle strap) is a very thin shoulder strap used to support clothing, while providing minimal shoulder straps over otherwise bare shoulders. It is commonly used in garments such as swimwear, camisoles, crop tops, bras, cocktail dresses, and evening gowns, so-named for its resemblance to the thin pasta strings called spaghetti.
Usage examples of "spaghetti strap".
The leather pants, red ankle boots, and the spaghetti strap halter weren't too far from what I usually wore on casual Fridays to tick off my boss, but put them on a street corner at night….
She wore jeans that looked so neat they must have been pressed, a white short-sleeved shirt that was sheer enough that she'd put a spaghetti strap tank top under it, and a brown leather purse almost as large as my carry-on bag.
Nathaniel helped me dress in the coolest jammies I had--a silky spaghetti strap nightshirt that would have been too revealing if I hadn't been so damn short.
She had on a little black Saint Laurent dress, and spaghetti strap heels, and her hair was all tousled and witchy.
She was wearing a short off-white, spaghetti strap sundress and beige, ankle-strap heels.